A guardrail system includes a guardrail, a support post, and a fastener joining the guardrail and the support post. The support post includes a hole receiving the fastener, a fastener retention mechanism, and a slot for the movement of the fastener during an impact. The fastener retention mechanism retains the fastener in the hole until a predetermined level of force is attained during an impact, after which the fastener is released and moves into the slot. Methods of moving the guardrail relative to the post are also provided.